The invention resides in a fuel injection system for an internal combustion engine with an injection pump providing for an initial fuel injection phase and a main fuel injection phase.
Such a fuel injection system is known for example from DE 37 22 265 A1. For the injection of fuel, a piston moving downwardly within an injection pump sucks in fuel by way of a fuel supply line. After closing of a control valve in the fuel injection pump, and with the communication through the line leading from the injection pump through the fuel injector interrupted by the closed fuel injection nozzle needle, the upwardly moving piston generates a high pressure in this line which is also designated high pressure space. As a result of the high pressure generated in this line, the injector's nozzle needle opens and fuel is injected into the combustion chamber of the internal combustion engine.
In addition to the control valve already referred to, the injection pump includes another control valve. One of the control valves is closed during the whole injection period, whereas the other control valve is opened for the termination of the fuel injection procedure.
The injection system described in this patent, however, has the disadvantage that the flow cross-section of the other control valve must be sufficiently large such that the injection procedure can be safely interrupted or, respectively, the initial injection can be terminated at maximum engine speed. However, such a large flow cross-section of the other valve results at a low engine speeds in a disadvantageous excessive pressure release from the high pressure space whereby cavities are formed which, in the end, lead to an unstable main fuel injection process.
Another fuel injection system for internal combustion engines is described in DE 36 01 710 A1. In this case, the flow cross-section of the electromagnetic valve is adjustable in at least two stages and the fuel injection volume is divided into an initial injection volume and a main injection volume. In order to achieve the initial injection, the valve is shortly closed and subsequently opened. It is then again closed for the main injection procedure and subsequently opened to terminate the main fuel injection phase.
With such an injection system, it is disadvantageous that the high pressure space is highly depressurized when the valve is opened at low engine speeds, because of the large flow cross-section of the valve whereby, as described above, cavities are formed which result in an unstable main injection process. Furthermore, during a non-ballistic operation of the magnetic valve, the initial injection volume generated by the pump operated at a relatively high speed is excessively large. Valve hammering occurring in the process, that is seating of the valve member on the valve seat at high speed, result in different injection volumes.
For the general state of the art of fuel injection systems, reference is further made to DE 31 47 467 C1 and to WO 89/00242 A1.
It is the object of the present invention to provide a fuel injection system for an internal combustion engine wherein excessive pressure release from the high pressure space at low engine speeds is avoided and a sufficiently small initial injection volume is provided at high engine speeds.